A communication system provides for the communication of data between a sending station and a receiving station. Communication of data between the communication stations of a communication system is a necessary aspect of modern society. And, the need to communicate data through the use of a communication system shall likely continue to be at least as essential in the future. A wide variety of different types of communication systems have been developed and are regularly utilized to effectuate many different types of communication services.
As technological advancements have permitted, new types of communication systems have been developed and implemented. Radio communication systems are exemplary of communication systems that have benefited from the technological advancements. Improvements to existing types of radio communication systems as well as new types of radio communication systems have been made possible as a result of the technological advancements.
Radio communication stations differ with conventional, wireline communication systems in that a communication path extending between sending and receiving stations includes, along at least a portion thereof, a radio link. Radio channels are defined upon the radio link, and the data is communicated upon one or more radio channels to the receiving station. A fixed, wireline connection is obviated for the portion of the communication path that utilizes the radio link.
Various advantages are provided through the use of a radio communication system. Installation and deployment costs, for example, of a radio communication system are sometimes less than the corresponding costs required to install and deploy a corresponding wireline counterpart. And, a radio communication system can be implemented as a mobile communication system in which one, or more, of the communication stations operable therein are permitted mobility, forming, as a result, a mobile communication system.
Many radio communication systems utilize digital communication techniques in which data that is to be communicated is first placed into digitized form and formatted, such as into data packets or frames, according to a selected formatting scheme. Once formatted, the data is modulated and communicated by a sending station to a receiving station. The data is communicated, for instance, in discrete bursts. When delivered to the receiving station, the modulated data is demodulated and the informational content of the data is recovered.
In an ideal communication system, the values of the data contained in each of the data packets or frames are of substantially identical values at the sending and receiving stations. That is, the values of the data are the same prior to transmission of the data to the receiving station as the values subsequent to their delivery to the receiving station. But, in an actual communication system, the values of the data are distorted and differ, sometimes significantly, when delivered to the receiving station in contrast to their corresponding values when transmitted by the sending station. If the values differ significantly, the informational content of the data might not accurately be able to be recovered.
Various communication conditions deleteriously affect the communication of the data, during its transmission by the sending station, to the receiving station. Multipath transmission conditions, for instance, cause the fading of the data during its transmission to the receiving station. Multipath transmission conditions are sometimes also referred to as fading conditions. The fading of the data alters the values of the data, or portions thereof, such that, when detected at the receiving station, the values of the data differ with corresponding values, when sent by the sending station. The fading conditions are sometimes modeled and categorized as being either quasi static flat fading or fast flat fading. Quasi static flat fading models a situation in which fading is flat in frequency and is constant during the duration of time during which a relevant portion of data is transmitted. In contrast, fast flat fading models a situation in which fading is flat in frequency but changes quickly between time periods. At a minimum, if the propagation distortion caused by fading is not properly corrected, the resultant communication quality levels of communications are, at a minimum, reduced.
Compensation is made for the propagation distortion introduced upon the data through the use of various techniques. By increasing the diversity of the data, the likelihood that the informational content of the data can be recovered is correspondingly increased.
Time diversity is one type of diversity that is sometimes increased. When data is time-encoded, for instance, the redundancy of the data is increased. Due to the increased redundancy, loss of portions of the data due to communication upon a communication channel that exhibits fading conditions is less likely to prevent the informational content of the data to be recovered.
Space diversity is another type of diversity that is sometimes increased. Space diversity, typically, refers to the utilization of more than one transmit antenna at a sending station at which to transduce the data that is to be communicated. The antennas are separated by selected separation distances to provide selected levels of correlation of the data communicated by the different ones of the antennas. Traditionally, the separation distances are selected to ensure that the data communicated by the respective transducers fade in an uncorrelated manner.
Multiple antennas are sometimes also used at the receiving station. Data detected at different ones of the antennas at the receiving station are also used to help compensate for the propagation distortion.
A communication system in which multiple transmit antennas and multiple receive antennas are utilized is sometimes referred to as an MIMO (multiple-input, multiple-output) communication system. The number of receive antennas is generally, but not necessarily, at least as great as the number of transmit antennas in an MIMO communication system.
In an MIMO communication system, independent data streams can be transmitted at different ones of the multiple transmit antennas. Communications in an MIMO system not only provide for improved communications upon non-ideal channels, but also permit the data throughput rate in such a communication system to be increased, generally corresponding to the increase in the number of transmit antennas. That is to say, the potential data throughput increase is linearly related with the number of transmit antennas that are utilized. And, to realize the potential data throughput increase permitted through the use of an MIMO system, the receiving station must also be able reliably to detect each of the individual data streams communicated by the sending station, in the presence of interference or fading conditions.
Space-time codes are sometimes used by which to encode the data that is to be communicated to increase the diversity at the receiving station by employing the multiple transmit antennas and encoding the data across the transmit antennas, in the time domain.
Both the space-time codes and the MIMO scheme usually benefit, performance-wise, if the antennas are uncorrelated or have low cross-correlation values. At the receiving station, the data is recovered due to the redundancy provided in time by the space-time codes and, respectively, the redundancy provided in space by the multiple receive antennas. Space-time coding does not require the use of multiple receive antennas of an MIMO system. But, conventionally, if the receiving station does not utilize the multiple receive antennas, the data throughput rate permitted in the system is reduced relative to the corresponding throughput rate permitted in a MIMO system that utilizes the multiple receive antennas.
When multiple receive antennas are utilized at the receiving station, relatively complex computational procedures are required to be performed at the receiving station to recover the informational content of the data received at the different ones of the receive antennas.
If a manner could be provided by which the data throughput rates permitted in a MIMO system that utilizes multiple receive antennas could be achieved while using only a single receive antenna, the benefits provided by a MIMO system, without the complexity associated with the recovery of data from the multiple receive antennas, would be achieved.
It is in light of this background information related to communication systems that utilize diversity techniques that the significant improvements of the present invention have evolved.